Filter mediums for producing exchangeable filter elements in hydraulic systems are known in many forms and typically comprise a multilayer filter non-woven having a support layer on one or both exterior sides, inflow or outflow sides, of the filter non-woven. Fluid to be cleaned, for example a hydraulic fluid, flows through the filter elements, with a significant pressure difference from the inflow to the outflow sides. To be able to withstand this pressure difference and also dynamic flow forces in the unfiltered fluid, the filter mediums, from which the appropriate filter elements are manufactured, have support layers, also called support structures. The support layers undergo strong pressure changing loads during operation of the filter elements and are made from metal fabric, in particular, fabric made of stainless steel wires or from plastic grids, for example.
The document EP 0 402 099 B1 discloses a reinforced textile material for use for fiber reinforced composite materials, comprising a plurality of warp threads composed of reinforced filament yarns, which extend parallel to each other in a web shape in the longitudinal direction of the reinforcing textile material. The warp threads are disposed in the transverse direction to the reinforced textile, in order to form a section composed of the warp threads having higher density and a section composed of the warp threads having lower density. A plurality of weft threads are composed of reinforced filament yarns, which extend parallel to each other in a web shape in one direction over the warp threads, and extend diagonal to the expansion direction of the warp threads.
With a filter medium disclosed in the document DE 10 2010 025 220 A1, a plastic grid, serving as a support structure, is formed from two groups of linear elements. Between the linear elements through-openings are delimited. One of the groups extends diagonally with a predetermined angle to the longitudinal and transverse directions of the web. The other groups extend parallel to the longitudinal or transverse direction of the web. A uniform shape stability and filtration characteristic of the filter medium are attained over the surface of the plastic grid due to the grid, or respectively screen-shaped arrangement of the linear elements.
The document EP 1 436 062 B1 relates to a filter element for fluids having a filter material and tissue-forming support structure supporting the filter material with respect to the through flow of the filter element, at least on the cleaned side. The support structure is manufactured from a plastic material and has electrically conductive elements. This support structure forms a support fabric that has both metal and plastic threads and lies flat against a filter material. The filter material and the support webs are folded in the shape of a star or pleated. The metal threads extend parallel to the folds of the support web.
A filter medium of the initially named type is known for example from the document, DE 10 2008 029 443 A1. The known filter medium is formed as a type of filter web composed of a first layer support fabric, a second layer as a protective non-woven, a third layer as a main non-woven, possibly a further layer of a subsequent further protective non-woven, and in any event, a fourth layer of a another support tissue. The support webs are composed of individual plastic threads. The linear elements, composed of plastic threads forming the plastic tissue, extend diagonally to the longitudinal and transverse direction of the filter web. The filter web is folded pleat-like into a filter element. The spacings between the linear elements serve as free through-openings for the flow through of the fluid to be filtered.
With the exception of the plastic grid according to the document DE 10 2010 025 220 A1 in which the plastic threads or individual linear elements are securely connected together via junction locations or intersections and yield a flow-through grid structure, the other aforementioned citations refer to metal and/or plastic fabric built as linear elements composed of warp and weft threads. Those treads or linear elements can move with respect to each other within the fabric composite material, such that the through-openings between the threads or linear elements are not constant. In particular, the through-openings can become closed due to the moved tissue composite material to impair both the support properties as well as the permeability of the known tissues serving as a support layer.